"The Physics of the Solar Pond" was described in some detail by Hershel Weinberger of the National Physical Laboratory of Israel in Solar Energy, Vol. VIII, No. 2, 1964, pages 45-56. According to this article, it has been found that solar radiation can effect a considerable temperature rise in non-convecting ponds of about a meter depth. Salt water at temperatures near 100.degree. C. can be withdrawn for utilization from the bottom of the ponds. The efficiency of the pond as a collector of solar energy is then greater than 20 percent and 4 percent of the incident energy can be converted by a Carnot engine to electrical energy. The pond is maintained non-convecting when there is a temperature gradient in the pond, with the bottom warmer then the top, by means of a salt concentration gradient that assures the greater density at the lower regions.
In many places of the world there exist extensive deposits of sodium sulfate. The deposits mainly consist of Mirabilite, or Glauber's salt, which is sodium sulfate, combined with 10 molecules of water. These 10 molecules of crystal-bound water account for 55.9% of the weight of the crystal and, therefore, it is very advantageous to remove the water prior to shipment. The crystals are monoclinic, white, transparent to opaque, and occur readily in nature. Tenardite, or anhydrous sodium sulfate, is defined as Na.sub.2 SO.sub.4 and is found sparsely in nature. It forms orthorombic and twin crystals. The transition temperature between the salts having a water content of 10 molecules and anhydrous form is about 32.degree. C.
One of the main consideration in order to purify and dehydrate the Glauber's salt to obtain the anhydrous sulfate (Tenardite), is the cost of energy required to remove the water present therein. The refining and dehydration of the salt in various commercial plants is based amongst others on the following equipment: rotary dryers, drum dryers, submerged combustion evaporators, multiple effect evaporators, flash dryers, tray dryers, fluidized bed dryers etc. These are relatively expensive and energy consuming equipment.